In a wireless communication system, a base station makes decisions regarding Modulation and Coding Scheme (MCS) selection and user equipment scheduling based on the quality of the data channel between the base station and the user equipment. To this end, the user equipment measures information about the channel quality and feeds back this information to the base station. This channel quality information is referred to as Channel Quality Indicator (CQI). CQI values can be defined in a number of ways. For example, CQI values may be defined by the Signal-to-Noise Ratio (SNR) measured by user equipment. CQI values may also be defined as the maximum MCS level supported by the measured SNR (i.e. the MCS level with the highest spectral efficiency and an acceptable error rate). An MCS level refers to such modulation information as the modulation order and code rate.
It may be desirable to reduce the feedback overhead between a base station and user equipment to maximize performance of a wireless communication system. In order to reduce the CQI feedback overhead, a CQI table can be utilized by the base station and user equipment. A CQI table contains CQI indices which are indicative of measured CQI values such as SNR. The user equipment then only needs to feed back CQI indices to the base station in order to provide the base station with information about channel quality. Sending CQI indices rather than actual CQI values can reduce the feedback overhead.
As mentioned above, CQI indices may be defined as the maximum MCS level supported by a measured SNR. However, the maximum MCS level supported for a given SNR depends on factors such as channel statistic and Multiple-Input Multiple-Output (MIMO) transmission mode. For a given SNR, the maximum MCS level supported in one channel type may not be the same maximum MCS level supported for another channel type. Thus, for different channel statistics and MIMO transmission modes, different CQI tables may be needed to fully exploit the system capacity.
CQI methods, such as those discussed above may be utilized in a 3rd Generation Partnership Project (3GPP) wireless communication system such as a Long Term Evolution (LTE) downlink system. Guidelines for generating CQI tables are discussed in 3GPP Specification TS 36.213, the entire contents of which are incorporated by reference. Section 7.2.3 of 3GPP Specification TS 36.213 discusses and describes the definition of CQI and CQI tables. Guidelines for generating CQI tables have been agreed upon in 3GPP RAN1 meeting #51. For example, one such guideline is that a CQI index is defined in terms of a code rate and modulation, such as Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16QAM), and 64-Quadrature Amplitude Modulation (64QAM).